A conventional motor drive apparatus for driving a motor is used in an electric power steering system, for example, which assists a steering torque of a vehicle and the like by a rotation torque of a motor. In this motor drive apparatus, AC power is generated from a DC power source by an inverter and supplied to the motor.
For example, in an electric power steering system according to JP 3884450 (JP 2006-21645A), a motor relay, which shuts off power supply from an inverter to a three-phase AC motor, is formed of semiconductor switching elements. In case that three semiconductor switching elements are used as the motor relay in addition to six semiconductor switching elements of the inverter, the motor drive apparatus necessarily becomes large in size.
In case that a semiconductor switching element is provided further as a power supply relay between a DC power source and an inverter for shutting off a power supply circuit, a motor drive apparatus further becomes large in size.